Allow passing arguments to PIL.Image.save().

[anntzer]

PR Summary

Closes #8530; more general than #8531 (this way we don't need to implement each and every kwarg separately).
Unfortunately, does not help with #5397 as png output does not go though pillow.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

👍 in principle, but needs a test.

[anntzer]

Test added. (Explicitly not testing compression for tiff as that requires pillow+libtiff and I'd rather write something independent of whether libtiff is installed.)
I realized we could also fix #5397 by having png-saving also go through pillow when pil_kwargs is not None, but that'll be a separate PR after this one.

[timhoffm]

Conditional on CI passing.

[efiring]

Looks good, but I see a test using pil_kwargs only for TIFF, not for Jpeg. Can the latter be added easily?

[anntzer]

@efiring That's because Pillow can be built with or without libjpeg support, and I don't want to bother checking what build variant we have (I'm not even sure what to check or what exception to catch -- if you know, please let me know); on the other hand, Pillow always does have tiff support (as that's pure python).